Algae Blooms in Lakes and Ponds

Blue-Green Algae

Blue-green algae, also known as cyanobacteria, can grow rapidly during the summer. The blooms thrive in slow-moving water bodies, such as lakes and ponds. Some cyanobacteria species can produce cyanotoxins during algal blooms which can be harmful at elevated levels if ingested by dogs, wildlife and humans, or during wading and other recreational contact with water. The toxicity of a cyanobacteria bloom is difficult to predict because a single species can have toxic and non-toxic strains, and toxic strains do not always produce toxins. 

With the exception of Union Reservoir, the City of Longmont does not test for blue-green algae regularly because of the many lakes and ponds in the City, but may monitor when a bloom is reported. The city has implemented a comprehensive monitoring and response plan at Union Reservoir and City staff perform visual inspections at McIntosh Lake. Because swimming and wading are prohibited in most Longmont lakes (except for Union Reservoir) the City does not anticipate conducting additional testing in those locations. However, the City of Longmont stresses that visitors and residents should exercise caution and keep children and pets out of the water in those areas or any areas where algae are observed.

Frequently Asked Questions

What do blue-green algal blooms look like?

According to the Colorado Department of Public Health and Environment, harmful algae blooms often have the following characteristics:

• May look like thick pea soup or spilled paint on the water’s surface.
• Can create a thick mat of foam along the shoreline.
• Usually are green or blue-green, although they can be other colors, including red or gold.
• Sometimes are made up of small specks or blobs floating just at or below the water’s surface.

Harmful algae blooms are NOT:

• Long, stringy bright green grass strands that feel either slimy or cottony.
• Mustard yellow (this is probably pollen).

What causes blue-green algae?

Blue-green algae thrive in warm, nutrient rich water. Water nutrients such as nitrogen and phosphorus help algae grow and support fish and other aquatic life. But too much nitrogen and phosphorus in the water allows blue-green algae to grow quickly and form blooms and scums. Blooms can occur anytime but are most common during hot, sunny weather and in slow-moving water bodies such as lakes. 

Is swimming allowed in bodies of water in City parks?

Swimming is not permitted in any park water bodies in the City except Union Reservoir per Longmont’s Park Regulations. The rule is based on State Health Department requirements because water quality monitoring is not conducted at any park water body except Union Reservoir.

• Union Reservoir is the only City pond where recreational contact, specifically swimming, is allowed.
• McIntosh Lake is not a City pond, but the City has a recreational lease. Carry-on, non-motorized boats only. This includes canoes, kayaks, bellyboats, sailboats and other small boats. This does not include inner tubes.

Are dogs allowed to go into bodies of water in City parks?

Current rules and regulations do not prohibit leashed pets from entering a water body in a City park or open space, with the exception of waters at Button Rock Preserve  However, pet owners should always exercise caution and keep pets out of the water if there is any question about the presence or absence of a harmful algae bloom.

Is blue-green algae harmful to humans?

Some algae blooms produce toxins that can cause illness in humans, pets and wildlife that come in contact with the algae. Humans can present a range of symptoms. Download the Colorado Department of Public Health and Environment’s Fact Sheet [PDF] about algae blooms to learn more.

How long does blue green algae last? Is blue green algae predictable?

The timing, intensity and duration of a blue-green algae bloom will vary from year to year based on nutrient availability, air and water temperatures, sunlight and wind velocity. Since these factors vary from year to year, blooms cannot be accurately predicted.

How can we get rid of harmful algae blooms?

We can’t eliminate blue-green algae from a lake — they are common in the overall algal community. What we really want to do is control their overall intensity and the frequency of the blooms. Since we can’t control the water temperature, the best thing we can do is to reduce the amount of nutrients getting into the lake. This can best be accomplished by reducing the amount of phosphorus and nitrogen from man-made sources such as lawn fertilizer, and runoff from cities, cultivated fields, feedlots and a myriad of other sources. Though a reduction of nuisance algal blooms will not be immediate, it is the best long-term solution to minimizing the frequency and intensity of algal blooms.

Can a blue-green algae bloom occur in the St. Vrain Creek or Left Hand Creek?

These algae prefer slow moving or stagnant water. Flowing streams like the St. Vrain and Left Hand Creek are far less susceptible to a bloom; however, as water levels decrease following spring runoff, stagnant pools may form along the banks of flowing streams. These pools could be susceptible to a bloom and should be avoided if there is any question about the presence of harmful algae.

What should I do if I or my pet is experiencing symptoms associated with harmful algae blooms?

If you believe that you or your pet has become ill because of harmful algae, seek medical attention. For questions about harmful algae health effects, contact the poison control center. If you believe that a pet or livestock has become ill because of harmful algae, contact your veterinarian immediately.

Where can I find more information about harmful algae blooms and pets?

Download this brochure developed by New York Sea Grant [PDF] to learn more information about how to keep your pets safe.

Is my drinking water safe?

The City of Longmont has good quality water that meets all State and Federal regulations. It obtains its source water from North St. Vrain Creek, Carter Lake and South St. Vrain Creek. These sources are high quality, protected and monitored extensively. Minimal human impact and low nutrient loading make them less susceptible to blue-green algal blooms. The City of Longmont Water Treatment Plant visually inspects source water on a daily basis and has procedures in place to deal with an algal bloom. Its treatment processes would be able to handle all but the most extreme of blue-green algal blooms. In 2018, the City of Longmont participated in an EPA study that included monitoring for cyanotoxins in its treated water. The cyanotoxins were not detected in any of the samples.